The fabrication of composite parts or items containing one or more fibrous reinforcements on one hand and a thermoplastic or thermohardenable (i.e., thermosettable) resin matrix on the other hand may be accomplished, for example, by Resin Transfer Molding (RTM) techniques. RTM consists of two stages: (i) fabricating a fiber preform in the shape of the desired finished item, and (ii) impregnating the preform with a thermoplastic or thermohardenable resin. The resin is injected or infused by aspiration and then thermocompressed to harden the resin after polymerization.
Preforms generally contain several superimposed sheets of fiber elements bound to each other by a binder in order to provide cohesion of the preform components and to allow handling of the preform. The preform typically contains only a small amount of binder, present solely to assure the binding of the fiber elements to each other. The resin necessary to fabricate a given composite part is added subsequently at the time the desired composite part is molded. The fiber elements can be either strands or cables, depending on the number of filaments or fibers. Most often, performs comprise superimposed unidirectional sheets such that the fiber elements are stretched parallel to each other in each of the sheets with the various unidirectional sheets extending in different directions.
Notably, for applications in the aeronautic, aerospace, and automobile domains, it is sometimes necessary to form performs having at least one portion which has an annular, an ellipsoidal, or a truncated cone shape such as in the construction of frames, portholes, reactor jets or intakes. The use of preforms comprising unidirectional sheets to fabricate such parts creates parts having mechanical characteristics that are not always adapted to their use, insofar as the reinforcement fiber elements that compose the fibrous sheets are not appropriately oriented with respect to the lines of curvature of the mechanical part along which the principal stresses are generally exerted.
In order to fabricate and obtain satisfactory mechanical properties for such preforms, whose shape follows at least one curved longitudinal generator line, it is necessary to place fiber element sheets such that the fiber elements are not parallel to the curved generator line. It is very difficult to produce sheets that provide a homogeneous covering without voids using this process. Indeed, the resulting mechanical properties are not satisfactory if the radial sheet does not cover the entire surface of the preform.